The present invention relates to a high permeability casting plaster and more particularly to a metal casting plaster of high permeability that allows any plastermold castings and in particular the simulated die casting of parts used primarily for the creation of prototype and short run production parts which will substantially replicate the appearance and function of conventionally die cast parts.
It will be appreciated by those skilled in the art that the use of gypsum-type materials in the manufacture of molds used for casting of metals is well known. The need for inexpensive simulated die casting is particularly important for the creation of prototype or short run production parts which can be tested prior to the outlay of large sums of money to develop a permanent steel die. To this end, there have been several attempts to provide such a plaster for allowing the creation of prototype and short run production parts.
One such attempt is disclosed by R. Reingen in U.S. Pat. No. 3,748,290. The Reingen patent briefly mentions the use of the addition of certain chemicals to a hemihydrate gypsum. The specific application of Reingen is directed toward ceramic molding. However, Reingen does mention that his method has had certain success with the manufacture of other molds, patterns, and pressed pieces. However, the Reingen patent does not create the thin wall casting that is required to develop prototypes.
U.S. Pat. No. 4,749,431 issued to Fassle et al. on June 7, 1988, discloses the use of alpha and beta hemihydrates that are added to glass fibers. However, this disclosure, too, does not describe the simulated die casting that is required to cast a prototype piece.
Several problems have plagued the aluminum/zinc alloy casting industry for many years. For example, the wall thickness of the metal castings vary due to irregularities in the surface of the mold. Also, turbulence in the molten metal caused by gases trapped in the mold cavity have caused poor metal wall structure. Further, low strength mold walls separate and crack under high temperatures or pressure, resulting in flashings or fins on the metal walls of the casting. Conventional casting plasters, when cured, have also lacked the uniform permeability necessary to provide uniform gas diffusion through and cooling of the molten metal during casting.
What is needed, then, is a high permeability metal casting plaster than can be used inexpensively to create simulated die castings. This needed plaster must produce a harder and smoother skin on the plaster mold. This plaster must also hold the foam cells in the plaster mold in uniform suspension. To prevent large, irregular foam cells from forming directly behind the skin, which causes the collapse of the mold wall under the weight of the molten alloy, the plaster must promote uniform density throughout the finished plaster mold. The plaster must also control cracking and micro-cracking of the mold surface that is caused by thermal expansion during drying and subsequent exposure to high temperature by the molten alloy. A plaster having these qualities and characteristics is presently lacking in the art.